INFO

problematic. Light tars like phenol chemically pollute bleed water of downstream condensers and aqueous scrubbers. Naphthalene is important as it is known to crystallise at the inlet of gas engines causing high maintenance costs.

Fouling of conventional water Naphthalene crystals in the scrubbers due to heavy tars gas engine control valve

Principle of OLGA The tar removal system of

OLGA is based on a multi stage scrubber in which gas is cleaned by special scrubbing oil. In the 1st loop of OLGA the gas is gently cooled down by scrubbing oil. Heavy tar particles condense and are collected, after which they are separated from the scrubbing oil and recycled to the gasifier. In the 2nd loop lighter tars are absorbed by scrubbing oil. In the absorber column the scrubbing oil is saturated by these light tars. This saturated oil is regenerated in a stripper. Hot air or steam is used to strip the tar out of the scrubbing oil. All heavy and light tars can be recycled to the gasifier where they are destructed and contribute to the energy efficiency and a tar waste stream is avoided!

Overview Tar is the Achilles heel for biomass and

waste gasification. Once the tar problem is solved, gasification has an important future for an independent and sustainable energy supply. The Energy research Centre in the Netherlands (ECN) tried almost every available tar removal system after which it was concluded that a new approach was necessary. From this research the OLGA technology was born. OLGA is a Patented ECN invention and an acronym for oil-gas scrubber. An oil is used to clean the gas. The scrubbing oil is reused and tars are recycled; energy is kept in the process and a tar waste stream is avoided! The tar problem The presence of tars in the

product gas is a big problem in the commercial utilisation of gasification product gas as a source of sustainable energy. Tar is formed in the gasifier and comprises a wide spectrum of organic compounds, generally consisting of several aromatic rings. Simplified, tars can be distinguished in heavy tars and light tars. Heavy tars already condense out as the gas temperature drops below 250-350°C and cause major fouling, efficiency loss and unscheduled plant stops. The tar dew point, i.e. the temperature at which tars start to condense, is a critical factor.

Light tars like phenol or naphthalene have less influence on the tar dew point, but are not less

G A S I F I C A T I O N O L G A T A R R E M O V A L

INFO

particles and aerosols are removed by the column as well as the downstream electrostatic precipitator (ESP). This ESP cleans itself by the tar/oil fraction it captures. The condensed tars and captured solid particles together with the scrubber oil are circulated. A recovery system in a bypass stream separates the heavy tars and the solids from this circulation loop. The heavy tar and dust mixture is recycled to the gasifier. After the collector and the ESP the gas is free of heavy tars and solids. The light tars with key components phenol and naphthalene are removed in the absorber. Absorption is a limited process, the absorption oil is saturated with tars. Therefore, the saturated oil is sent to a stripper in which the tars are stripped off by heated air or steam. As this stripper uses the combustion / fluidisation air required by the gasifier these light tars are recycled and destroyed in the gasifier. Gas no longer suffers from tar related problems and can be dewatered, further cleaned if necessary and utilized. Because all captured solids, heavy & light tars are recycled to the gasifier, OLGA is in principle a waste free system. It is important to realise that the captured tars contain a considerable amount of energy. This is not wasted but kept in the system and contributing to the energy output of the plant.

Process description Product gas contains solids, tars and inorganic impurities. In principle the mixing of dust, tar and water must be avoided. The philosophy of OLGA is based on dew point control. The tar (TDP) and water (WDP) dew points are shown in the figure below and the related equipment is shown in the figure on the previous page.

Before entering the OLGA tar removal system the product gas is cooled and the coarse particles are removed by a cyclone. The first OLGA column accepts gas above the tar dew point (400-500°C) and cools it down to a temperature close but safely above the water dew point. Heavy tars are removed by condensation. Fine

W W W . D A H L M A N . N L | T . + 3 1 ( 0 ) 1 0 5 9 9 1 1 1 1 | E . D A H L M A N @ D A H L M A N . N L M A Y 2 0 1 3

Features Benefits Removal of tars without water condensation No poisoned water,

minimization of waste water treatment costs

High tar removal efficiency, very low tar dew point, in comparison with thermal cracking

Product gas suitable for gas engines, gas turbines and catalytic upgrading

Tar recycle, heavies and lights No tar waste streams, all energy recovered, resulting in lower operating costs

More reliable and less vulnerable than a catalytic tar cracker

Increased system stability and availability

Compatible with most industrial gasifiers A robust solution giving operational freedom to gasifiers

Conden

sation

Te

mp

era

ture

ºC

Dew points & process choices

TDP ± 350 ˚C

Water dew point ± 60 ˚C

Tar dew point < 10 ˚C

Abso

rptio

n

Co

ole

r

OLGA

Separation of:

tars & fine particles

Pa

rtic

le

se

pa

ratio

n

T = 850 °C

Actual

temperature

Water Quench,

condenser & scrubber

(inorganics)

WDP ± 30 ºC